PD-1 regulates neural damage in oligodendroglia-induced inflammation

Pathogenic inflammation in the CNS of mice carrying human PLP1 mutations

Progressive forms of multiple sclerosis lead to chronic disability, substantial decline in quality of life and reduced longevity. It is often suggested that they occur independently of inflammation. Here we investigated the disease progression in mouse models carrying PLP1 point mutations previously found in patients displaying clinical features of multiple sclerosis. These mouse models show loss-of-function of PLP1 associated with neuroinflammation; the latter leading to clinically relevant axonal degeneration, neuronal loss and brain atrophy as demonstrated by inactivation of the recombination activating gene 1. Moreover, these pathological hallmarks were substantially amplified when we attenuated immune regulation by inactivation of the programmed cell death-1 gene. Our observations support the view that primary oligodendroglial abnormalities can evoke pathogenically relevant neuroinflammation that drives neurodegeneration, as observed in some forms of multiple sclerosis but also in other, genetically-mediated neurodegenerative disorders of the human nervous system. As many potent immunomodulatory drugs have emerged during the last years, it is tempting to consider immunomodulation as a treatment option not only for multiple sclerosis, but also for so far non-treatable, genetically-mediated disorders of the nervous system accompanied by pathogenic neuroinflammation.

Melanocortin-1 receptor activation is neuroprotective in mouse models of neuroinflammatory disease

In inflammation-associated progressive neuroinflammatory disorders, such as multiple sclerosis (MS), inflammatory infiltrates containing T helper 1 (T_H1) and T_H17 cells cause demyelination and neuronal degeneration. Regulatory T cells (T_reg) control the activation and infiltration of autoreactive T cells into the central nervous system (CNS). In MS and experimental autoimmune encephalomyelitis (EAE) in mice, T_reg function is impaired. We show that a recently approved drug, Nle⁴-d-Phe⁷-α-melanocyte-stimulating hormone (NDP-MSH), induced functional T_reg, resulting in amelioration of EAE progression in mice. NDP-MSH also prevented immune cell infiltration into the CNS by restoring the integrity of the blood-brain barrier. NDP-MSH exerted long-lasting neuroprotective effects in mice with EAE and prevented excitotoxic death and reestablished action potential firing in mouse and human neurons in vitro. Neuroprotection by NDP-MSH was mediated via signaling through the melanocortin-1 and orphan nuclear 4 receptors in mouse and human neurons. NDP-MSH may be of benefit in treating neuroinflammatory diseases such as relapsing-remitting MS and related disorders.

Neuroinflammation as modifier of genetically caused neurological disorders of the central nervous system: Understanding pathogenesis and chances for treatment

Genetically caused neurological disorders of the central nervous system (CNS) are usually orphan diseases with poor or even fatal clinical outcome and few or no treatments that will improve longevity or at least quality of life. Neuroinflammation is common to many of these disorders, despite the fact that a plethora of distinct mutations and molecular changes underlie the disorders. In this article, data from corresponding animal models are analyzed to define the roles of innate and adaptive inflammation as modifiers and amplifiers of disease. We describe both common and distinct patterns of neuroinflammation in genetically mediated CNS disorders and discuss the contrasting mechanisms that lead to adverse versus neuroprotective effects. Moreover, we identify the juxtaparanode as a neuroanatomical compartment commonly associated with inflammatory cells and ongoing axonopathic changes, in models of diverse diseases. The identification of key immunological effector pathways that amplify neuropathic features should lead to realistic possibilities for translatable therapeutic interventions using existing immunomodulators. Moreover, evidence emerges that neuroinflammation is not only able to modify primary neural damage-related symptoms but also may lead to unexpected clinical outcomes such as neuropsychiatric syndromes.

Clinically relevant intronic splicing enhancer mutation in myelin proteolipid protein leads to progressive microglia and astrocyte activation in white and gray matter regions of the brain

INTRODUCTION

Mutations in proteolipid protein (PLP), the most abundant myelin protein in the CNS, cause the X-linked dysmyelinating leukodystrophies, Pelizaeus-Merzbacher disease (PMD) and spastic paraplegia type 2 (SPG2). Point mutations, deletion, and duplication of the PLP1 gene cause PMD/SPG2 with varying clinical presentation. Deletion of an intronic splicing enhancer (ISEdel) within intron 3 of the PLP1 gene is associated with a mild form of PMD. Clinical and preclinical studies have indicated that mutations in myelin proteins, including PLP, can induce neuroinflammation, but the temporal and spatial onset of the reactive glia response in a clinically relevant mild form of PMD has not been defined.

METHODS

A PLP-ISEdel knockin mouse was used to examine the behavioral and neuroinflammatory consequences of a deletion within intron 3 of the PLP gene, at two time points (two and four months old) early in the pathological progression. Mice were characterized functionally using the open field task, elevated plus maze, and nesting behavior. Quantitative neuropathological analysis was for markers of astrocytes (GFAP), microglia (IBA1, CD68, MHCII) and axons (APP). The Aperio ScanScope was used to generate a digital, high magnification photomicrograph of entire brain sections. These digital slides were used to quantify the immunohistochemical staining in ten different brain regions to assess the regional heterogeneity in the reactive astrocyte and microglial response.

RESULTS

The PLP-ISEdel mice exhibited behavioral deficits in the open field and nesting behavior at two months, which did not worsen by four months of age. A marker of axonal injury (APP) increased from two months to four months of age. Striking was the robust reactive astrocyte and microglia response which was also progressive. In the two-month-old mice, the astrocyte and microglia reactivity was most apparent in white matter rich regions of the brain. By four months of age the gliosis had become widespread and included both white as well as gray matter regions of the brain.

CONCLUSIONS

Our results indicate, along with other preclinical models of PMD, that an early reactive glia response occurs following mutations in the PLP gene, which may represent a potentially clinically relevant, oligodendrocyte-independent therapeutic target for PMD.

Programmed cell death 1 inhibits inflammatory helper T-cell development through controlling the innate immune response

Programmed cell death 1 (PD-1) is an inhibitory coreceptor on immune cells and is essential for self-tolerance because mice genetically lacking PD-1 (PD-1(-/-)) develop spontaneous autoimmune diseases. PD-1(-/-) mice are also susceptible to severe experimental autoimmune encephalomyelitis (EAE), characterized by a massive production of effector/memory T cells against myelin autoantigen, the mechanism of which is not fully understood. We found that an increased primary response of PD-1(-/-) mice to heat-killed mycobacteria (HKMTB), an adjuvant for EAE, contributed to the enhanced production of T-helper 17 (Th17) cells. Splenocytes from HKMTB-immunized, lymphocyte-deficient PD-1(-/-) recombination activating gene (RAG)2(-/-) mice were found to drive antigen-specific Th17 cell differentiation more efficiently than splenocytes from HKMTB-immunized PD-1(+/+) RAG2(-/-) mice. This result suggested PD-1's involvement in the regulation of innate immune responses. Mice reconstituted with PD-1(-/-) RAG2(-/-) bone marrow and PD-1(+/+) CD4(+) T cells developed more severe EAE compared with the ones reconstituted with PD-1(+/+) RAG2(-/-) bone marrow and PD-1(+/+) CD4(+) T cells. We found that upon recognition of HKMTB, CD11b(+) macrophages from PD-1(-/-) mice produced very high levels of IL-6, which helped promote naive CD4(+) T-cell differentiation into IL-17-producing cells. We propose a model in which PD-1 negatively regulates antimycobacterial responses by suppressing innate immune cells, which in turn prevents autoreactive T-cell priming and differentiation to inflammatory effector T cells.

Biliary obstruction results in PD-1-dependent liver T cell dysfunction and acute inflammation mediated by Th17 cells and neutrophils

Biliary obstruction is a common clinical problem that is associated with intrahepatic inflammation and impaired immunity. PD-1 is well known to mediate T cell dysfunction but has been reported to promote and attenuate acute inflammation in various injury models. With the use of a well-established murine model of BDL, we studied the effects of intrahepatic PD-1 expression on LTC function, inflammation, and cholestasis. Following BDL, PD-1 expression increased significantly among LTCs. Increased PD-1 expression following BDL was associated with decreased LTC proliferation and less IFN-γ production. Elimination of PD-1 expression resulted in significantly improved proliferative capacity among LTC following BDL, in addition to a more immunostimulatory cytokine profile. Not only was LTC function rescued in PD-1(-/-) mice, but also, the degrees of biliary cell injury, cholestasis, and inflammation were diminished significantly compared with WT animals following BDL. PD-1-mediated acute inflammation following BDL was associated with expansions of intrahepatic neutrophil and Th17 cell populations, with the latter dependent on IL-6. PD-1 blockade represents an attractive strategy for reversing intrahepatic immunosuppression while limiting inflammatory liver damage.

Progressive multiple sclerosis: pathology and pathogenesis

Major progress has been made during the past three decades in understanding the inflammatory process and pathogenetic mechanisms in multiple sclerosis (MS). Consequently, effective anti-inflammatory and immunomodulatory treatments are now available for patients in the relapsing-remitting stage of the disease. This Review summarizes studies on the pathology of progressive MS and discusses new data on the mechanisms underlying its pathogenesis. In progressive MS, as in relapsing-remitting MS, active tissue injury is associated with inflammation, but the inflammatory response in the progressive phase occurs at least partly behind the blood-brain barrier, which makes it more difficult to treat. The other mechanisms that drive disease in patients with primary or secondary progressive MS are currently unresolved, although oxidative stress resulting in mitochondrial injury might participate in the induction of demyelination and neurodegeneration in both the relapsing-remitting and progressive stages of MS. Oxidative stress seems to be mainly driven by inflammation and oxidative burst in microglia; however, its effects might be amplified in patients with progressive MS by age-dependent iron accumulation in the brain and by mitochondrial gene deletions, triggered by the chronic inflammatory process.

Neuroinflammation by cytotoxic T-lymphocytes impairs retrograde axonal transport in an oligodendrocyte mutant mouse

Mice overexpressing proteolipid protein (PLP) develop a leukodystrophy-like disease involving cytotoxic, CD8+ T-lymphocytes. Here we show that these cytotoxic T-lymphocytes perturb retrograde axonal transport. Using fluorogold stereotactically injected into the colliculus superior, we found that PLP overexpression in oligodendrocytes led to significantly reduced retrograde axonal transport in retina ganglion cell axons. We also observed an accumulation of mitochondria in the juxtaparanodal axonal swellings, indicative for a disturbed axonal transport. PLP overexpression in the absence of T-lymphocytes rescued retrograde axonal transport defects and abolished axonal swellings. Bone marrow transfer from wildtype mice, but not from perforin- or granzyme B-deficient mutants, into lymphocyte-deficient PLP mutant mice led again to impaired axonal transport and the formation of axonal swellings, which are predominantly located at the juxtaparanodal region. This demonstrates that the adaptive immune system, including cytotoxic T-lymphocytes which release perforin and granzyme B, are necessary to perturb axonal integrity in the PLP-transgenic disease model. Based on our observations, so far not attended molecular and cellular players belonging to the immune system should be considered to understand pathogenesis in inherited myelin disorders with progressive axonal damage.

A novel hypothesis: regulatory B lymphocytes shape outcome from experimental stroke

Although inflammatory immune cells clearly contribute to the development of middle cerebral artery occlusion (MCAO) in mice, the failure to block neutrophil-associated injury in clinical stroke trials has discouraged further development of immunotherapeutic approaches. However, there is renewed interest in a possible protective role for regulatory T- and B-cells that can suppress inflammation and limit central nervous system damage induced by infiltrating pro-inflammatory cells. Our failure to implicate CD4(+)FoxP3(+) T-cells in limiting brain lesion volume after MCAO turned our focus towards regulatory B-cells known to mediate protection against other inflammatory CNS conditions. Our results clearly demonstrated that B-cell deficient mice developed larger infarct volumes, higher mortality and more severe functional deficits compared to wild-type mice, and had increased numbers of activated T-cells, macrophages, microglial cells, and neutrophils in the affected brain hemisphere. These MCAO-induced changes were completely prevented in B-cell-restored mice after transfer of highly purified WT B-cells but not IL-10-deficient B-cells. Our novel observations are the first to implicate IL-10-secreting B-cells as a major regulatory cell type in stroke and suggest that enhancement of regulatory B-cells might have application as a novel therapy for this devastating neurologic condition.

Human brain endothelial cells endeavor to immunoregulate CD8 T cells via PD-1 ligand expression in multiple sclerosis

Background

Multiple sclerosis (MS), an inflammatory disease of the central nervous system (CNS), is characterized by blood-brain barrier (BBB) disruption and massive infiltration of activated immune cells. Engagement of programmed cell death-1 (PD-1) expressed on activated T cells with its ligands (PD-L1 and PD-L2) suppresses T cell responses. We recently demonstrated in MS lesions elevated PD-L1 expression by glial cells and absence of PD-1 on many infiltrating CD8 T cells. We have now investigated whether human brain endothelial cells (HBECs), which maintain the BBB, can express PD-L1 or PD-L2 and thereby modulate T cells.

Methods

We used primary cultures of HBECs isolated from non-tumoral CNS tissue either under basal or inflamed conditions. We assessed the expression of PD-L1 and PD-L2 using qPCR and flow cytometry. Human CD8 T cells were isolated from peripheral blood of healthy donors and co-cultured with HBECs. Following co-culture with HBECs, proliferation and cytokine production by human CD8 T cells were measured by flow cytometry whereas transmigration was determined using a well established in vitro model of the BBB. The functional impact of PD-L1 and PD-L2 provided by HBECs was determined using blocking antibodies. We performed immunohistochemistry for the detection of PD-L1 or PD-L2 concurrently with caveolin-1 (a cell specific marker for endothelial cells) on post-mortem human brain tissues obtained from MS patients and normal controls.

Results

Under basal culture conditions, PD-L2 is expressed on HBECs, whilst PD-L1 is not detected. Both ligands are up-regulated under inflammatory conditions. Blocking PD-L1 and PD-L2 leads to increased transmigration and enhanced responses by human CD8 T cells in co-culture assays. Similarly, PD-L1 and PD-L2 blockade significantly increases CD4 T cell transmigration. Brain endothelium in normal tissues and MS lesions does not express detectable PD-L1; in contrast, all blood vessels in normal brain tissues are PD-L2-positive, while only about 50% express PD-L2 in MS lesions.

Conclusions

Our observations suggest that brain endothelial cells contribute to control T cell transmigration into the CNS and immune responses via PD-L2 expression. However, such impact is impaired in MS lesions due to downregulation of endothelium PD-L2 levels.

Programmed death-1 pathway limits central nervous system inflammation and neurologic deficits in murine experimental stroke

BACKGROUND AND PURPOSE

Evaluation of infarct volumes and infiltrating immune cell populations in mice after middle cerebral artery occlusion strongly implicates a mixture of both pathogenic and regulatory immune cell subsets that affect stroke outcome. Our goal was to evaluate the contribution of the well-described coinhibitory pathway, programmed death (PD)-1, to the development of middle cerebral artery occlusion.

METHODS

Infarct volumes, functional outcomes, and effects on infiltrating immune cell populations were compared in wild-type C57BL/6 versus PD-1-deficient mice after 60 minutes middle cerebral artery occlusion and 96 hours reperfusion.

RESULTS

The results clearly demonstrate a previously unrecognized activity of the PD-1 pathway to limit infarct volume, recruitment of inflammatory cells from the periphery, activation of macrophages and central nervous system microglia, and functional neurological deficits. These regulatory functions were associated with increased percentages of circulating PD-ligand-1 and PD-ligand-2 expressing CD19(+) B-cells in blood, the spleen, and central nervous system with the capacity to inhibit activation of inflammatory T-cells and central nervous system macrophages and microglial cells through upregulated PD-1.

CONCLUSIONS

Our novel observations are the first to implicate PD-1 signaling as a major protective pathway for limiting central nervous system inflammation in middle cerebral artery occlusion. This inhibitory circuit would likely be pivotal in reducing stroke-associated Toll-like receptor-2- and Toll like receptor-4-mediated release of neurotoxic factors by activated central nervous system microglia.

The majority of infiltrating CD8 T lymphocytes in multiple sclerosis lesions is insensitive to enhanced PD-L1 levels on CNS cells

Central nervous system (CNS) cells locally modulate immune responses using numerous molecules that are not fully elucidated. Engagement of programmed death‐1 (PD‐1), expressed on activated T cells, by its ligands (PD‐L1 or PD‐L2) suppresses T‐cell responses. Enhanced CNS PD‐1 and PD‐L1 expression has been documented in inflammatory murine models; however, human CNS data are still incomplete. We determined that human primary cultures of astrocytes, microglia, oligodendrocytes, or neurons expressed low or undetectable PD‐L1 under basal conditions, but inflammatory cytokines significantly induced such expression, especially on astrocytes and microglia. Blocking PD‐L1 expression in astrocytes using specific siRNA led to significantly increased CD8 T‐cell responses (proliferation, cytokines, lytic enzyme). Thus, our results establish that inflamed human glial cells can express sufficient and functional PD‐L1 to inhibit CD8 T cell responses. Extensive immunohistochemical analysis of postmortem brain tissues demonstrated a significantly greater PD‐L1 expression in multiple sclerosis (MS) lesions compared with control tissues, which colocalized with astrocyte or microglia/macrophage cell markers. However, more than half of infiltrating CD8 T lymphocytes in MS lesions did not express PD‐1, the cognate receptor. Thus, our results demonstrate that inflamed human CNS cells such as in MS lesions express significantly elevated PD‐L1, providing a means to reduce CD8 T cell responses, but most of these infiltrating immune cells are devoid of PD‐1 and thus insensitive to PD‐L1/L2. Strategies aimed at inducing PD‐1 on deleterious activated human CD8 T cells that are devoid of this receptor could provide therapeutic benefits since PD‐L1 is already increased in the target organ. © 2011 Wiley‐Liss, Inc.

Enhanced antiviral T cell function in the absence of B7-H1 is insufficient to prevent persistence but exacerbates axonal bystander damage during viral encephalomyelitis

The T cell inhibitory ligand B7-H1 hinders T cell-mediated virus control, but also ameliorates clinical disease during autoimmune and virus-induced CNS disease. In mice infected with gliatropic demyelinating coronavirus, B7-H1 expression on oligodendroglia delays virus control, but also dampens clinical disease. To define the mechanisms by which B7-H1 alters pathogenic outcome, virus-infected B7-H1-deficient (B7-H1(-/-)) mice were analyzed for altered peripheral and CNS immune responses. B7-H1 deficiency did not affect peripheral T or B cell activation or alter the magnitude or composition of CNS-infiltrating cells. However, higher levels of IFN-γ mRNA in CNS-infiltrating virus-specific CD8 T cells as well as CD4 T cells contributed to elevated IFN-γ protein in the B7-H1(-/-) CNS. Increased effector function at the single-cell level was also evident by elevated granzyme B expression specifically in virus-specific CNS CD8 T cells. Although enhanced T cell activity accelerated virus control, 50% of mice succumbed to infection. Despite enhanced clinical recovery, surviving B7-H1(-/-) mice still harbored persisting viral mRNA, albeit at reduced levels compared with wild-type mice. B7-H1(-/-) mice exhibited extensive loss of axonal integrity, although demyelination, a hallmark of virus-induced tissue damage, was not increased. The results suggest that B7-H1 hinders viral control in B7-H1 expressing glia cells, but does not mediate resistance to CD8 T cell-mediated cytolysis. These data are the first, to our knowledge, to demonstrate that B7-H1-mediated protection from viral-induced immune pathology associated with encephalomyelitis resides in limiting T cell-mediated axonal bystander damage rather than direct elimination of infected myelinating cells.

Ectopic T-cell specificity and absence of perforin and granzyme B alleviate neural damage in oligodendrocyte mutant mice

In transgenic mice overexpressing the major myelin protein of the central nervous system, proteolipid protein, CD8+ T-lymphocytes mediate the primarily genetically caused myelin and axon damage. In the present study, we investigated the cellular and molecular mechanisms underlying this immune-related neural injury. At first, we investigated whether T-cell receptors (TCRs) are involved in these processes. For this purpose, we transferred bone marrow from mutants carrying TCRs with an ectopic specificity to ovalbumin into myelin mutant mice that also lacked normal intrinsic T-cells. T-lymphocytes with ovalbumin-specific TCRs entered the mutant central nervous system to a similar extent as T-lymphocytes from wild-type mice. However, as revealed by histology, electron microscopy and axon- and myelin-related immunocytochemistry, these T-cells did not cause neural damage in the myelin mutants, reflecting the need for specific antigen recognition by cytotoxic CD8+ T-cells. By chimerization with bone marrow from perforin- or granzyme B (Gzmb)-deficient mice, we demonstrated that absence of these cytotoxic molecules resulted in reduced neural damage in myelin mutant mice. Our study strongly suggests that pathogenetically relevant immune reactions in proteolipid protein-overexpressing mice are TCR-dependent and mediated by the classical components of CD8+ T-cell cytotoxicity, perforin, and Gzmb. These findings have high relevance with regard to our understanding of the pathogenesis of disorders primarily caused by genetically mediated oligodendropathy.

Accelerated course of experimental autoimmune encephalomyelitis in PD-1-deficient central nervous system myelin mutants

It is assumed that the onset and course of autoimmune inflammatory central nervous system (CNS) disorders (eg, multiple sclerosis) are influenced by factors that afflict immune regulation as well as CNS vulnerability. We challenged this concept experimentally by investigating how genetic alterations that affect myelin (primary oligodendrocyte damage in PLPtg mice) and/or T-cell regulation (deficiency of PD-1) influence both the onset and course of an experimental autoimmune CNS inflammatory disease [MOG(35-55)-induced experimental autoimmune encephalomyelitis (EAE)]. We observed that double pathology was associated with a significantly earlier onset of disease, a slight increase in the neurological score, an increase in the number of infiltrating cells, and enhanced axonal degeneration compared with wild-type mice and the respective, single mutant controls. Double-mutant PLPtg/PD-1(-/-) mice showed an increased production of interferon-gamma by CNS immune cells at the peak of disease. Neither PD-1 deficiency nor oligodendropathy led to detectable spread of antigenic MHC class I- or class II-restricted epitopes during EAE. However, absence of PD-1 clearly increased the propensity of T lymphocytes to expand, and the number of clonal expansions reliably reflected the severity of the EAE disease course. Our data show that the interplay between immune dysregulation and myelinopathy results in a stable exacerbation of actively induced autoimmune CNS inflammation, suggesting that the combination of several pathological issues contributes significantly to disease susceptibility or relapses in human disease.